The release of cell contents of fresh rye-grass (Lolium perenne L.) during digestion in dairy cows: effect of the intracellular constituents, season and stage of maturity

The objective of the trial was (i) to measure the release kinetics of intracellular constituents (ICs) of fresh grass during eating and after a meal in dairy cows fed rye-grass (Lolium perenne L.), (ii) to quantify the effect of the season (spring versus autumn) and the stage of maturity on the release of ICs, and (iii) to dissociate the effects of chewing during rumination from the effects of ruminal processes (microbial degradation of cell walls, ruminal mixing, osmotic shocks) in the release of ICs. In each season, four dairy cows fitted with a large ruminal cannula were fed indoors, during two 3 h periods, with fresh grass cut at 23 and 57 days of regrowth in spring, and at 34 and 68 days in autumn. The rumen of each animal was evacuated every 3 h during daylight (0, 3, 6 and 9 h after the beginning of the meal). Subsamples of ruminal contents were rinsed carefully and their IC contents compared with the IC contents of grass. To dissociate the effect of ruminal processes, ingestive boli were incubated in nylon bags in the rumen and rinsed at the same times as the rumen evacuations. luble carbohydrates were quickly released. More than 85% were released by the end of the meal and all the soluble carbohydrates disappeared from forage particles 6 h after the meal. The release kinetics did not vary very much between free sugars and fructans. The proportions of chlorophyll released were far lower. Only 45% of the chlorophyll was released by the end of the meal and an additional 45% was released 6 h later. The release of soluble carbohydrates was only marginally affected by the stage of maturity and season. The release of chlorophyll and fructans was lower for young grass in autumn. The release of chlorophyll was quicker in spring than in autumn. lease of soluble carbohydrates was almost complete before rumination began. The release kinetics of chlorophyll in ingestive boli incubated in nylon bags in autumn was parallel to the release kinetics measured in the ruminal contents, thus showing that chewing during rumination is not a major process in the release of ICs. However, the effect of rumination may have been more important in spring as suggested by the positive correlation between the unit ruminating time and the proportions of chlorophyll released. A high weight of ruminal contents may enhance the release of chlorophyll by increasing the ruminal pressure. results clearly show that the release kinetics differs between chlorophyll and the other ICs and suggest that ingestive mastication and ruminal processes other than rumination are of major importance in the release of ICs.